The present invention relates to removal of protective coatings from components exposed to high temperatures, such as components of a gas turbine engine.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. In this regard, materials containing silicon as a matrix material or a reinforcing material, are currently being used for high temperature applications, such as for combustor and other hot section components of gas turbine engines, because of the good capacity of these silicon materials to operate at higher temperatures.
Such high-temperature materials, such as, for example, ceramics, alloys, and intermetallics, offer attractive properties for use in structures designed for service at high temperatures in such applications as gas turbine engines, heat exchangers, and internal combustion engines, for example. However, the environments characteristic of these applications often contain reactive species, such as water vapor, which at high temperatures may cause significant degradation of the material structure. For example, water vapor has been shown to cause significant surface recession and mass loss in silicon-bearing materials. The water vapor reacts with the structural material at high temperatures to form volatile silicon-containing species, often resulting in unacceptably high recession rates.
Components that are exposed to these high temperatures, such as a component within a gas turbine engine, typically include protective coatings. For example, turbine blades, turbine vanes, and blade outer air seals typically include one or more coating layers that protect the component from erosion, oxidation, corrosion or the like to thereby enhance durability and/or maintain efficient operation of the engine.
Environmental barrier coatings (EBC's) are applied to silicon-bearing materials and other material susceptible to attack by reactive species, such as high temperature water vapor. EBC's provide protection by prohibiting contact between the environment and the surface of the material. EBC's applied to silicon-bearing materials, for example, are designed to be relatively stable chemically in high-temperature, water vapor-containing environments. One exemplary conventional EBC system, as described in U.S. Pat. No. 6,410,148, comprises a silicon or silica bond layer applied to a silicon-bearing substrate; an intermediate layer comprising mullite or a mullite-alkaline earth aluminosilicate mixture deposited over the bond layer; and a top layer comprising an alkaline earth aluminosilicate deposited over the intermediate layer. In another example, U.S. Pat. No. 6,296,941, the top layer is a yttrium silicate layer rather than an alumino silicate. An exemplary bond layer, or bond coat is disclosed in U.S. Pat. No. 6,299,988.
Though significant advances have been made with barrier coating materials and processes for producing both the environmentally-resistant bond coat and the barrier coating, there is the inevitable requirement to remove and replace the barrier coating and bond coat under certain circumstances. For example, removal may be necessitated by erosion or impact damage to the ceramic layer during engine operation, or by a requirement to repair certain features such as the tip length of a turbine blade. Removal of the barrier coatings and/or the bond coat may also be necessitated during component manufacturing to address such problems as defects in the coating, handling damage and the need to repeat noncoating-related manufacturing operations which require removal of the barrier coating and/or bond coat, e.g., electrical-discharge machining (EDM) operations.
The current state-of-the-art repair methods often result in removal of the entire barrier coating system, i.e., both the barrier coatings and bond coat, after which the bond coat and barrier coatings must be redeposited. Prior art abrasive techniques for removing barrier coatings have generally involved grit blasting, vapor honing and glass bead peening, each of which is a slow, labor-intensive process that erodes the barrier coatings and bond coat, as well as the substrate surface beneath the coating. With repetitive use, these removal processes eventually destroy the component by reducing the wall thickness of the component.
Therefore, what is needed, inter alia, are new and improved methods for removing barrier coatings and bond coats rapidly and without damage to an underlying substrate, for example a ceramic matrix composite substrate, such as a gas turbine engine component.